1. Field of the Invention
The present invention relates to angular position measurement, more specifically, to an electromagnetic method and apparatus for measuring angular position, rotational speed, or displacement, of a movable target.
2. Description of the Prior Art
The usefulness of the application of an RF or microwave field for the measurement of angular position is recognized by the prior art. Such devices can operate with either RF or microwave excitation. When an electromagnetic field is excited near the rotating part of this type of device, the parameters of the electromagnetic field, such as resonant frequency, phase, or amplitude, vary with a change of angular position of the rotating part. The electromagnetic field parameters may be converted into a desired type of indication, including angle, angular displacement, angular speed, rotation frequency, and so on. In particular, such art is exemplified in U.S. Pat. No. 3,939,406 “Sensing Rotational Speed by Amplitude Modulating a Continuous Microwave Signal,” /F. W. Chapman, F. E. Jamerson, and N. L. Muench, 1971, disclosing an electrodynamic sensor including two cavity resonators, one connected to a microwave generator, the other connected to a microwave receiver, the two cavity resonators are placed near a muff installed on the rotating part, the muff has identical slots in a cylindrical surface along a generatrix, and positioned periodically in the angular direction. The rotation of the slots is influenced by the angular position of the muff, and leads to a change in the electromagnetic connection between the resonators and, as a result, to the amplitude modulation of the signal passing from the microwave generator to the receiver. The modulation frequency is proportional to rotational speed.
A general discussion, see V. A. Viktorov, B. V. Lunkin and A. S. Sovlukov, “Radio-Wave measurements” [in Russian], Moscow: Energoatomizdat, 1989, pp. 148-153, states that a microwave resonator is placed near the rotating part, of which a surface electrodynamic property (the “electrodynamic profile”) changes in the azimuth direction, and the resonator's frequency has a direct correlation to the angular position of the rotating part.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, see J. R. Pierce, “Traveling-Wave Tubes” D. Van Nostrand Company, Inc., Princeton, N.J., 1950. These waves are electromagnetic waves propagating in one direction with a phase velocity vp that is smaller than the velocity of light c in vacuum. The relation c/vp is called slowing or deceleration, and is designated as N, or the slowing factor. In most practical applications, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors, for example, into a helix, or radial spiral (prior art), which increases the path length traveled by the wave. The coiled conductor is called the “impedance conductor,” the other is called a “screen conductor.” Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures, in which both conductors are coiled and have the configuration of mirror images turned by 180° relative to the plane of symmetry, see Yu. N. Pchelnikov, “Comparative Evaluation of the Attenuation in Microwave Elements Based on a Spiral Slow-Wave System,” Soviet Journal of Communication Technology and Electronics, Vol 32, #11, 1987, pp. 74-78.
Slow-wave structure-based sensing elements are known in the art, see V. V. Annenkov, Yu. N. Pchelnikov “Sensitive Elements Based on Slow-Wave Structures” Measurement Techniques, Vol. 38, #12, 1995, pp. 1369-1375. The slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensing elements and this enables one, by using the advantages of electrodynamic structures, to operate such a device at relatively low frequencies for a given size of sensing element. Lower frequencies are more convenient for generation and are more convenient for primary conversion of the information signal, but must be sufficiently high to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also helps to increase the accuracy and sensitivity of the measurements. The slowing of the electromagnetic wave also leads to energy concentration in the transverse and longitudinal directions, that results in an increase in sensitivity, proportional to the slowing factor N, see Yu. N. Pchelnikov, “Nontraditional Application of Surface Electromagnetic Waves” Abstract Book, First World Congress on Microwave Processing, 1997, pp. 152-153.
Prior Art in the same field as the present invention includes “Electromagnetic Method of the Angular Displacement Monitoring” using sensing elements based on slow-wave structures and a moving target with a changing electrodynamic profile (U.S. Pat. No. 6,393,912 B2, Yu. N. Pchelnikov and D. S. Nyce). A rotary position sensing method based on slow-wave structures is taught in this patent. The present invention, however, teaches a different and novel apparatus forming an angular position sensor with improved measurement capability and performance.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior art methods and the present invention use two or more stationary elements, placed near a rotating part, having an “electrodynamic profile.” The stationary elements are connected to a measuring circuit comprising an RF or microwave signal generator which is used to excite an electromagnetic field. A change in position of the rotating part causes a shift in the characteristics of the electromagnetic field in the stationary elements. See V. A. Viktorov, B. V. Lunkin and A. S. Sovlukov, “Radio-Wave measurements” [in Russian], Moscow: Energoatomizdat, 1989, pp. 148-153.
Devices according to the prior art exhibit several problems which are overcome by the present invention. Some of the previous methods have low accuracy, sensitivity, and resolution at relatively low frequency, increasing only with an increase in frequency. However, the increase in frequency is accompanied by an increase in electromagnetic losses, such losses causing a loss of accuracy of the measurement. It is also known that the higher the frequency is, the higher the cost of electronics. Apparatus according to the prior art are relatively complex and require a complex set of electronics to convert the informative parameter into a usable reading. Thus, there is a need in the art for improving apparatus for measuring angular position, and related values, with lower cost, having good sensitivity, and high resolution.